Fixes for PEP451 import loaders and pytest 5.x

- pytest 5.x drops python2 compatibility and therefore only implements PEP 451
- pytest 5.x made the repr of `ExcInfo` less confusing (fixed tests depending
  on the old format)

examples/tutorial/tests/test_db.py

@@ -12,7 +12,7 @@ def test_get_close_db(app):
     with pytest.raises(sqlite3.ProgrammingError) as e:
         db.execute('SELECT 1')
 
-    assert 'closed' in str(e)
+    assert 'closed' in str(e.value)
 
 
 def test_init_db_command(runner, monkeypatch):